The subcellular trafficking of tracer-tagged GLUT 4 between the plasma membranes and low-density microsomes of rat adipose cells has been studied. Cell-surface GLUT 4 have been initially tracer-tagged in the insulin-stimulated state with a [3H)-bis-mannose. The initial experiments show that insulin does not alter the half-time for GLUT 4 endocytosis but instead increases the rate of exocytosis. Additional data suggest that the cells' entire complement of GLUT 4 is involved in the recycling process. Finally, detailed time-course data suggest that there may be plasma membrane intermediate states in the GLUT4 trafficking pathways. Peptides from the alpha 1 domain of the major histocompatibility complex class 1 antigen (MHC class 1) enhance cellular glucose uptake above that of maximal insulin stimulation, prolong the effect of insulin, and inhibit insulin receptor internalization in rat adipose cells. Based on the new data here, we now propose that MHC class 1 molecules may be involved in regulation of the internalization process of cell surface integral membrane proteins such as the glucose transporter, IGF-II receptor, and insulin receptor. We have also found that EGF in combination with certain MHC class I-derived peptides is insulinomimetic and that this effect is independent of insulin receptor activity. In order to obtain reliable kinetic constants for the two glucose transporter isoforms found in insulin responsive tissues, each transporter was expressed in Xenopus oocytes by the injection of mRNA encoding rat GLUT1 or GLUT4. The 3-O- methylglucose kinetic data indicate that, at low substrate concentrations, the catalytic efficiency of GLUT4 is significantly greater than GLUT1. Extrapolation to mammalian systems suggests that GLUT4 is responsible for virtually all of the hexose uptake in insulin responsive targets, particularly in the presence of hormones. In studies of the effects of K+ depletion, we observed an inhibition of GLUT 4 internalization which is entirely analogous to the effects on IGFII/Man-6-P receptor cycling strongly suggesting the involvement of a coated pit mechanism in the recycling of GLUT4 transporters. An inactive conformation of GLUT 4 has also been detected in plasma membranes from insulin-stimulated cells which is enhanced by K+ depletion without a corresponding increase in transport activity suggesting a limit in the adipose cells' capacity to promote active GLUT4 transporters.